A user equipment (UE) has to move according to a user's movement in a mobile communication system unlikely a wired communication system. Accordingly, mobility of the user equipment should be considered in the mobile communication system. In other words, in the mobile communication system, a network is supposed to support capabilities for providing with the user equipment services above certain levels even in case that the user equipment moves from an area to other area. More particularly, when the user equipment goes away from a first network entity with which the user equipment has a connection and moves into a new second network entity, the network should perform a procedure like handover, etc for shifting an access point of the user equipment from the first network entity to the second network entity. The first and second network entities may be base stations or other higher network nodes. When the first and second network entities are the base stations, the first network entity can be called a serving or center base station and the second network entity a target base station.
A WCDMA system in accordance with the 3rd generation partnership project (3GPP) technical specification supports three radio link control (RLC) modes to provide a variety of quality of services (QoS) for different kinds of services, which are a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM). Each RLC mode is carried out by a corresponding entity of an RLC layer. Since the quality of service (QoS) supported by each RLC mode is different each other, operating processes and detailed functions of each RLC mode also differs.
The acknowledged mode is different from the transparent mode and the unacknowledged mode in that a receiving side transmits a reception acknowledgement in response to data received from a transmitting side. Namely, the receiving side transmits status report to inform the transmitting side of whether data blocks transmitted from the transmitted side are received successfully. The transmitting side retransmits to the receiving side data blocks for which negative reception acknowledgement are received. An object of the acknowledged mode is to support error-free data transmission. The acknowledged mode is usually used for transmitting non-real time packet data like TCP/IP data, etc and radio resource control (RRC) messages for which reception acknowledgements are necessary.
An acknowledged mode (AM) entity of a radio link control (RLC) layer has a complicated structure due to the retransmission function described above. The AM entity performs a variety of functions such as flow control with transmitting and receiving windows, polling for a transmitting side to request status information to a peer RLC entity of a receiving side, status report for the receiving side to transmit status report to a peer RLC entity of the transmitting side, etc. For those functions, a status protocol data unit (PDU) for delivering status information and a piggyback function by which a status PDU is inserted into a data PDU are supported by the RLC AM entity. Further, the RLC AM entity supports a reset PDU for requesting re-establishment of all operations and parameters to a counterpart AM entity when the RLC AM entity finds an important error during operation and a reset ACK PDU for responding to the reset PDU. A plurality of protocol parameters, status variables and timers are necessary for the AM RLC entity to perform the above-described functions. A PDU like a status PDU or reset PDU, which is used for controlling data transmission in the AM RLC entity is called a control PDU and a PDU for delivering user data is called a data PDU.
In case that a user equipment changes a network entity like a base station with which a connection is established due to a reason such as handover, etc, data loss and unnecessary repeated data transmission should be limited within a minimum range during the changing procedure. For instance, when a user equipment acting in the acknowledged mode performs handover from a source base station to a target base station, the source base station is supposed to transmit data blocks to be transmitted to the user equipment to the target base station. In this case, a transmission buffer or a retransmission buffer in the AM RLC entity of the source base station is storing data blocks which has not been transmitted to the user equipment at all and data blocks which has been transmitted to the user equipment but has received negative reception acknowledgements from the user equipment. Accordingly, when the source base station delivers data blocks for the user equipment to the target base station, efficient methods are required to avoid data loss or unnecessary repeated data transmission. Further, a method of new status report for the user equipment is needed for supporting efficient data transfers from the source base station to the target base station.